A lithium secondary battery having high electromotive force that comprises a nonaqueous electrolyte and utilizes oxidation and reduction of lithium has recently been used as one of new type high output and high energy density batteries.
In such lithium secondary batteries, a carbon material such as graphite, coke, and the like, capable of occluding and releasing lithium ion, is commonly used as a material for the negative electrode.
As the nonaqueous solvent to be used for the nonaqueous electrolyte, ethylene carbonate, propylene carbonate, butylene carbonate, sulfolan, γ-butyrolactone, ethylmethyl carbonate, and the like, have conventionally been used alone or in various combinations thereof.
γ-Butyrolactone having a high boiling point and excellent heat stability has recently been attracting attention for use as the nonaqueous solvent to improve the safety of the lithium secondary battery.
However, when γ-butyrolactone is used as the nonaqueous solvent, a surface film having excellent mobility of lithium ion cannot be formed on the surface of the carbon material used for the negative electrode and charge and discharge become difficult. γ-Butyrolactone is decomposed as a side reaction and storage characteristics of the battery are deteriorated.
It has been proposed to add ethylene carbonate capable of forming a surface film having excellent mobility of lithium ion on the surface of the carbon material to a nonaqueous solvent including γ-butyrolactone.
If an amount of ethylene carbonate added to the nonaqueous solvent including γ-butyrolactone is not sufficient, a surface film having excellent mobility of lithium ion is not formed on the surface of the carbon material. On the other hand, if an amount of ethylene carbonate added to the nonaqueous solvent including γ-butyrolactone is too great, heat stability of the nonaqueous electrolyte is reduced and safety of the lithium secondary battery cannot be sufficiently improved.
It has also recently been proposed to add vinyl ethylene carbonate to a nonaqueous solvent to improve charge discharge efficiency of the lithium secondary battery, and storage characteristics and charge discharge cycle characteristics at a high temperature (see, for example, Japanese Laid-open Publication No. 2001-6729).
However, even if vinyl ethylene carbonate is added to the nonaqueous solvent, discharge characteristics of the lithium secondary battery still cannot be sufficiently improved.